This invention relates to a storage container for hazardous material, especially heat-generating and/or radiant material, such as nuclear fuel rods and the like, which needs to be safely stored for a short or long times, such as for brief periods or decades or even centuries.
Storage of hazardous material in storage containers according to the invention may be for so-called ultimate storage but is primarily intended for interim storage, that is, storage in cases in which it may be required to process, recycle or otherwise handle the hazardous material after it has been stored for a longer or shorter time, such as during the time when the material is awaiting or under transport from one place to another.
WO91/05351 discloses a system for submerged offshore storage of hazardous materials, especially radioactive materials. This system includes primary storage units in the form of generally cylindrical storage bodies having a central cavity for receiving the hazardous material, and a secondary storage unit in the form of a substantially larger concrete structure adapted to be lowered to a storage position on the seabed to receive a large number of the primary storage units. Both the primary storage units and the secondary storage unit have buoyancy tanks which can be filled with water or evacuated as desired, so that the units can be transported on the water surface to the storage site and lowered to the seabed and when necessary caused to resurface.
WO96/21932 discloses a prior art storage container of the kind indicated initially, which comprises an elongate generally cylindrical concrete body having an elongate, sealable central interior cavity for accommodating the hazardous material and a liquid coolant in which the hazardous material is immersed, the cavity comprising a generally cylindrical storage section concentric with the concrete body and at least one expansion chamber in fluid communication with the storage section. This storage container may serve as the primary storage unit in the above-mentioned prior art storage system, being then provided with suitable flotation means, or other systems for interim or ultimate storage of the hazardous material.
The prior art storage container disclosed in WO96/21932 includes a conduit system, embedded in the concrete, for natural circulation of a liquid coolant, typically water, filling the storage section in which a canister holding the hazardous material is placed. This conduit system transports heat from the central storage section to the surrounding outer parts of the concrete body so that the heat may be dissipated to a medium surrounding the concrete body, such as a body of water in which the storage container is submerged. It is necessary then that the storage container be in a position at the storage site such that it is substantially vertical so that the natural circulation of the liquid coolant may be as effective as possible.
Because of the embedded conduit system and the required radiation absorption capability, the diameter of the concrete body of the prior art storage container is large. As a consequence, it is difficult to transport the storage container once the hazardous material has been introduced in it and needs to be cooled. In effect, it is very difficult, and may even be may be impracticable, to transport the prior art storage container on a standard railway car or other standard ground-transport vehicle.
A primary object of the present invention is to provide a storage container of the kind indicated initially which is suited for use in cases where some cooling is required but the cooling requirement is not very demanding, so that it can be met by means of a cooling arrangement which does not include a special fluid conduit system for transferring heat from the central cavity to the exterior surface of the storage container. Such a reduced cooling requirement exists in many cases, such as during transport or short-term storage of individual nuclear fuel rods.
The background prior art relevant to the invention includes a storage container of the kind disclosed in WO96/21932, namely a storage container of the kind comprising an elongate generally cylindrical concrete body having an elongate, sealable central interior cavity for accommodating the hazardous material and a cooling liquid in which the hazardous material is immersed, the cavity comprising a generally cylindrical storage section concentric with the concrete body and at least one expansion chamber in fluid communication with the storage section.
In accordance with the invention, the above-indicated primary object is achieved with a storage container of the just-mentioned kind in which the expansion chamber is located axially outside one end of the storage section and extends radially past the circumference of the storage section.
In a storage container constructed in accordance with the invention it is possible to confine the liquid coolant to the central cavity, so that a conduit or other passage system allowing the liquid coolant to circulate between the central cavity and the outer part of the concrete body can be dispensed with, and still ensure that the liquid coolant always completely fills the storage section of the cavity and can expand and contract as required in response to temperature variations, regardless of the orientation of the storage container.
Accordingly, unlike the prior art storage container, the storage container of the invention need not necessarily be in an upright position during the storage period (which may be several years even in the case of interim storage). Instead, it may be in the position which is the most practical in each individual case, such as in a horizontal position during transport. By suitable dimensioning of the expansion chamber and suitable filling of the central cavity with a liquid coolant in connection with the introduction of the hazardous material and the sealing of the cavity, a complete filling of the storage section with the liquid coolant can always be ensured, regardless of the orientation or position of the storage container. Thereby an adequate heat transfer from all points of the hazardous material to the concrete body can be ensured.
Because of the position of the expansion chamber axially outside one end of the storage section, the diameter of the concrete body and thus of the entire storage container can be minimised. Even when the hazardous material is spent nuclear fuel rods, it therefore is often possible to restrict the diameter of the storage container to 120 cm, for example, so that two storage containers can be placed side by side on a railway car of standard width.